It is previously known to use one or more cylinders in diesel engines as compressor cylinders by providing each of the cylinders with an extra exhaust valve and equipping the engine injection system with devices through which the fuel supply can be cut off to the engine cylinders functioning as compressor cylinders. Such a compressor arrangement can i.a. be designed to supplement the ordinary engine unit air compressor, which normally supplies the vehicle compressed air system when its capacity is not sufficient for some reason, or it can function as the sole compressed air source for the vehicle.
SE 9003735-9 describes an internal combustion engine of the type described by way of introduction. The extra east valve is hydraulically operated with the air of a hydraulic pump with control valves. When the engine cylinder with the extra exhaust valve is to work as a compressor, the valve is set in the open position and the fuel supply via the associated unit injector is cut off by holding its spill valve open. The exhaust valve is kept constantly open as long as the compressor function is to be maintained, and re-entry of air via the extra exhaust valve is prevented during the intake stroke by means of a non-return valve disposed in the exhaust channel.
An arrangement of this type functions satisfactorily when the engine is to work as a compressor at low load, for example, during engine braking when driving downhill. If an extra air supplement is required even when the engine is loaded, several problems will arise, however. The vibrations arising when an engine cylinder is completely decoupled, can be so great that the vehicle gearbox in particular will be subjected to unacceptable wear, which reduces its useful life to less than half of the normal useful life. If the engine is turbocharged, there is lost not only a sixth of the normal power (in a six-cylinder engine with one cylinder decoupled) but as much as half of the power can be lost due to the loss of charge air pressure which occurs when the gas flow to the turbine of the turbocompressor is reduced An additional problem is that the high air pressure in the cylinder in combination with the constriction of the non-return valve generates very high temperatures in the non-return valve, placing heavy requirements on its design to make its function and useful life acceptable.